A typical optically amplified undersea cable system employs a number of optical repeaters and thus requires a power supply for operation. For conventional systems each repeater requires circa 30V at 1 A, and the system may contain as many as 200 repeaters on a typical 6000 km trans-Atlantic application. To power the ensemble of repeaters a series connection technique is chosen using a constant current supply and earth return. Typically cable resistances are circa 1 Ohm/km and as such a total voltage would be 6 kV for the cable resistive losses and 6 kV for the repeaters of the above example. Voltage clamping in each repeater is used to extract operational current and bypass the excess—allowing for any repeater ageing or line current fluctuation, whilst maintaining a nominally constant voltage within the repeater for its control circuitry.
The use of a local earth return at the Power Feeding Equipment (PFE), allows for greater power efficiency than by using an isolated return current cable. For instance a typical earth may be 10 Ohms so dropping 10V at each station if the line current is 1 A, where-as a return cable would be 6000 Ohm dropping 6 kV total, entailing a higher supply system voltage and additional costs associated with a return cable, and also there is the cost of additional copper
The example described gives a total line voltage of 12 kV. Such a substantial voltage is undesirable due to additional insulation requirements and stresses on the system elements. FIG. 1 illustrates a typical implementation, where here two PFE's are installed, one supplying +6 kV and the other −6 kV, each at 1 A. Thus half the maximum 12 kV is rendered on any system element. In addition a single PFE failure may be overcome by temporarily increasing the voltage of the remaining one to maintain system operation until repair may be undertaken.
Submarine cable is typically coaxial containing a power member surrounded by insulation and then an optional outer earthed screen which in turn will be in contact with seawater. The structure typically has a capacitance of 0.2 uF/km and characteristic impendence of around 28 Ohms. These features have to be carefully considered in the design, particularly with high voltage with regards to damage protection of repeaters from cable faults or current surges from shore end lightning strikes.
The large line voltages discussed above lead to significant stress on repeater components. Components capable of dealing with such voltages are typically large, and repeaters containing them are correspondingly large. This is a significant expense when considering undersea repeaters.
For example, conventionally, repeater circuitry is protected from current surges by Zener diodes connected in parallel to the control circuitry of the repeater and also by the provision of air cored coil inductors. However, these components are necessarily large (in view of the voltages and possible surge currents involved) and are unsuitable for the provision of miniaturised repeaters.